Electrochemical battery cells that use a fluid, such as oxygen or other gas(es), from outside the cell as an active material to produce electrical energy, such as air-depolarized, air-assisted and fuel cell battery cells, can be used to power a variety of electronic devices. For example, air containing oxygen enters into an air-depolarized or air-assisted cell, where it can be used as, or can recharge, the positive electrode active material. The oxygen reduction electrode promotes the reaction of the oxygen with the cell electrolyte and, ultimately the oxidation of the negative electrode active material with the oxygen. The material in the oxygen reduction electrode that promotes the reaction of oxygen with the electrolyte is often referred to as a catalyst. However, some materials used in oxygen reduction electrodes are not true catalysts because they can be at least partially reduced, particularly during periods of relatively high rate discharge. They can also be recharged by an outside power source.
One type of air-depolarized cell is a zinc/air cell. This type of cell uses zinc as the negative electrode active material and has an aqueous alkaline (e.g., KOH) electrolyte. Manganese oxides that can be used in zinc/air cell air electrodes are capable of electrochemical reduction in concert with oxidation of the negative electrode active material, particularly when the rate of diffusion of oxygen into the air electrode is insufficient. These manganese oxides can then be reoxidized by the oxygen during periods of lower rate discharge or rest. Some of these manganese dioxides as well as the zinc negative electrode material can be recharged by power supplied from outside the zinc air or fuel cell.
Another type of air-depolarized cell is a lithium/air cell. This type of cell uses lithium as the negative electrode active material and has an aqueous or a organic liquid or solid electrolyte. Manganese oxides that can be used in lithium/air cell electrodes are capable of electrochemical reduction in concert with oxidation of the negative electrode active material, particularly when the rate of diffusion of oxygen into the air electrode is insufficient. These manganese oxides can then be reoxidized by the oxygen during periods of lower rate discharge or rest.
Air-assisted cells are hybrid cells that contain consumable positive and negative electrode active materials as well as an oxygen reduction electrode. The positive electrode can sustain a high discharge rate for a significant period of time, but through the oxygen reduction electrode, oxygen can only partially but significantly recharge the positive electrode during periods of lower or no discharge, so oxygen can be used for a substantial portion of the total cell discharge capacity. This means the amount of positive electrode active material put into the cell can be reduced and the amount of negative electrode active material can be increased to increase the total cell capacity in the standard volume for each size cell. Examples of air-assisted cells are disclosed in U.S. Pat. No. 6,383,674 and U.S. Pat. No. 5,079,106.
An advantage of air-depolarized, air-assisted and fuel cells is their high energy density, since at least a portion of the active material of at least one of the electrodes comes from or is regenerated by a fluid (e.g., a gas) from outside the cell. For zinc air cells another advantage is that they can be recharged for a number of cycles depending on previous discharge history and recharging voltage and current characteristics.
A disadvantage of these cells is that the maximum discharge rates they are capable of can be limited by the rate at which oxygen can enter the oxygen reduction electrode. In the past, efforts have been made to increase the rate of oxygen entry into the oxygen reduction electrode and/or control the rate of entry of undesirable gases, such as carbon dioxide, that can cause wasteful reactions. The cell performance can also depend on the rate of water entry or loss, which in turn depends on the relative water vapor partial pressures outside and inside the cell. The reaction products and increased water can fill void space in the cell intended to accommodate the increased volume of discharge reaction products or dry the cell out, respectively. Examples of these approaches can be found in U.S. Pat. Nos. 6,558,828; 6,492,046; 5,795,667; 5,733,676; U.S. Patent Publication No. 2002/0150814; and International Patent Publication No. WO 02/35641. However, changing the diffusion rate of one of these gases generally affects the others as well.
At higher discharge rates, it is more important to get sufficient oxygen into the oxygen reduction electrode, but during periods of lower discharge rates and periods of time when the cell is not in use, the importance of minimizing CO2 and water diffusion increases. To provide an increase in air flow into the cell only during periods of high rate discharge, fans have been used to force air into cells (e.g., U.S. Pat. No. 6,500,575), but fans and controls for them can add cost and complexity to manufacturing, and fans, even micro fans, can take up valuable volume within individual cells, multiple cell battery packs and devices. They also increase the continuous parasitic current required.
Another approach that has been proposed is to use microvalves to control the amount of air entering the cells (e.g., U.S. Pat. No. 6,641,947 and U.S. Patent Publication No. 2003/0186099).
Yet another approach has been to use a water impermeable membrane between an oxygen reduction electrode and the outside environment having flaps that can open and close as a result of a differential in air pressure, e.g., resulting from a consumption of oxygen when the battery is discharging (e.g., U.S. Patent Publication No. 2003/0049508).
Additional approaches utilizing microvalves to control the amount of gas entering a cell are set forth in U.S. Pat. Nos. 5,304,431; 5,449,569; 5,541,016; and 5,837,394; incorporated herein by reference, wherein the microvalves are formed utilizing a silicon or semiconductor substrate and etching, deposition and micromachining processes.
Microactuators are further described in U.S. Pat. Nos. 4,969,938; 5,069,419; 5,271,597; and publications such as “Fluister: semiconductor microactuator” described in Instruments and Apparatus News (IAN), October 1993, p. 47, and Electronic Design, Nov. 1, 1993, p. 3.